A gas turbine generally includes a compressor, a combustion system, and a turbine section. Within the combustion system, air and fuel are combusted to generate a heated gas. The heated gas is then expanded in the turbine section to drive a load.
Historically, combustion systems employed diffusion combustors. In a diffusion combustor, fuel is diffused directly into the combustor where it mixes with air and is burned. Although efficient, diffusion combustors are operated at high peak temperatures, which creates relatively high levels of pollutants such as nitrous oxide (NOx).
To reduce the level of NOx resulting from the combustion process, dry low NOx combustion systems have been developed. These combustion systems use lean pre-mixed combustion, which pre-mixes air and fuel to create a relatively uniform air-fuel mixture before directing the mixture into the combustion zone. The mixture is then combusted at relatively lower temperatures, generating relatively lower levels of NOx.
One combustor suited for lean, pre-mixed combustion is a two-stage combustor of the type disclosed in U.S. Pat. No. 4,292,801, entitled “Dual Stage-Dual Mode Low NOx combustor.” Such a combustor includes two combustion chambers positioned adjacent to each other. One of the combustion chambers is in communication with a number of primary fuel nozzles, while a second combustion chamber is in communication with a secondary fuel nozzle. The distinct nozzles permit introducing air and fuel into the combustion chambers in staged modes. In a pre-mixing mode, for example, a lean mixture of air and fuel is created in the first combustion chamber, which is then combusted in the second combustion chamber at a relatively lower, controlled peak temperature, reducing NOx production.
Although such combustion systems achieve lower levels of NOx emissions, the fuel nozzles may be relatively likely to experience undesirable flame conditions, such as flashback or auto-ignition. Flashback denotes the upstream propagation of a flame from an expected location in the combustion chamber into the fuel nozzle, while auto-ignition denotes the unexpected ignition of the air-fuel mixture directly in the fuel nozzle itself. Regardless of the source of the flame, the fuel nozzle may tend to “hold” the flame, which may damage the fuel nozzle or other portions of the gas turbine. To address this problem, combustion systems are normally designed to reduce the occurrence of auto-ignition, flashback and flameholding.
Recently, alternatives fuels have been investigated for use with gas turbines, which may improve efficiency, lower pollutant emissions, or both. For example, synthesis gases (“syngas”) are alternative fuels derived from sources such as coal. These and other alternative fuels may have a relatively high hydrogen content, which may be relatively reactive. The reactivity of such fuels improves the efficiency of the combustor, but exacerbates the risk for undesirable flame events such as flashback, auto-ignition, and flame holding.
Flame events may be particularly likely to occur in the secondary fuel nozzle of a two-stage combustion system. Because the secondary nozzle is not suited for use with syngas and other high reactivity fuels, the fuel flexibility of the system is limited.
From the above, it is apparent that a need exists for a dry low NOx combustion system that includes a secondary fuel nozzle suited for use with alternative fuels.